I'll happily chime in to promote traveling in Canada!
We live in Oakville, Ontario along the shores of Lake Ontario about a 30 minute drive from downtown Toronto and about an hours drive from Niagara Falls. Plenty to do in both areas to keep families busy for several days each.
Both my husband and I grew up in Northern Ontario so we have covered a fair bit of this province in our travels. With our kids we have also traveled to Vancouver, British Columbia; Québec; Prince Edward Island; Nova Scotia and New Brunswick. This summer we will be visiting Newfoundland as well as the other Atlantic provinces. We have even briefly visited the Yukon territory while on an Alaskan cruise. We need to get them to the western provinces soon to see the Rockies!
Everywhere that we have traveled in Canada has been very family-friendly. My kids love the East coast - especially Prince Edward Island. We visited the island a number of times when my older daughter was little and she has very fond memories. She is actually more excited about returning there this summer than she was about our trip to Paris in March!